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How many times have you played a card game with friends and asked for a few new cards in hopes of winning? It’s a simple, bullish exchange. 

As you think about triathlons and racing, wouldn’t it be great to be able to exchange the percentage of fast-twitch and slow-twitch muscle fibers in preparation for your “A” race?  Think about it.  You could load up on more fast-twitch muscle fibers for your next sprint event or go for a higher percentage of slow-twitch muscle fibers for your next Ironman distance event.

Unfortunately, this isn’t a card game where you can exchange the cards you have been dealt.  It is important to understand that when you think about the composition of your skeletal muscle fibers, you can thank your parents.

Yes, that is right.  Your parents’ genetics were passed on to you.  Your percentage of fast-twitch and slow-twitch muscle fibers cannot be changed.  However, it is imperative  to understand the value and importance of activating and/or stimulating the muscle fibers you have been given by your parents, via specific training routines.

  • Stimulating fast glycolytic muscle fibers (the first type of fast-twitch muscle fibers) requires short bursts of 8-15 seconds in all three disciplines.  These short bursts require a great deal of energy.  These are the muscle fibers that are needed at the start of the swim, the end of the swim, T1, short climbs on the bike, T2, and surges during the run.  Incorporating these short bursts in training will transfer to higher-quality short bursts during competition.
  • Stimulating fast oxidative muscle fibers (the second type of fast-twitch muscle fibers) requires interval training, fartlek training, and build-ups. (Fast oxidative muscle fibers are the primary contributor when you increase your intensity for a period of 45-90 seconds. These muscles fibers will contribute to the first few or last minutes of your swim, when you want to pass someone on the bike portion, and possibly help you accelerate with 400 yards to go to the finish line.) Just like the fast glycolytic muscles fibers, these muscles fibers must be stimulated during training in all three disciplines.  Interval training, fartlek training, and build-ups should be incorporated into your weekly training routine.

keep-calm-and-love-genetics-19How about the slow twitch muscle fibers?  These are important too and serve a major purpose.  Slow-twitch muscle fibers are the major players for endurance events.  When you are exercising aerobically or in Zones 1 or 2, the slow-twitch muscle fibers are doing their job and keep you moving for several hours.  Their fuel source is fat and much more economical as compared to larger volumes of carbohydrate or glycogen that is needed for fast-twitch muscle fibers.  


  • Maximizing your slow-twitch muscle fibers requires you to train for a longer duration of time and at a lower intensity.  One of the major benefits of the base-training phase of your annual training plan, which is primarily made up of low-intensity training sessions, is the utilization of these fibers. One long swim, one long bike, and one long run should be incorporated into your weekly training routine.

Even though most of us would like to change the percentage of fast-twitch and slow-twitch muscle fibers we were born with, shift your thinking by maximizing what you have been given.  Do the best with what you have.  Training sessions that incorporate specific main sessions or sets for each type of muscle fiber will increase the likelihood of better race results, despite the distance. Moreover, understanding the fuel requirements for each type will help you as you plan out your race nutrition and strategy.

Contributing Author: Jason McFaul

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